Photoacoustic image generation apparatus

ABSTRACT

An insert having a photoacoustic wave generation portion, an acoustic wave detection unit that detects photoacoustic waves and reflected acoustic waves, an acoustic image generation unit that generates a B mode acoustic image on the basis of the reflected acoustic waves, a photoacoustic image generation unit that generates a photoacoustic image on the basis of the photoacoustic waves, an image output unit that outputs a display image obtained by synthesizing the B mode acoustic image and the photoacoustic image, and a control unit that controls a detection frequency of the photoacoustic waves used in a case where the photoacoustic image is generated in the photoacoustic image generation unit on the basis of a resolution of the B mode acoustic image forming the display image are included.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No.PCT/JP2018/007066 filed on Feb. 27, 2018, which claims priority under 35U.S.C § 119(a) to Japanese Patent Application No. 2017-064579 filed onMar. 29, 2017. Each of the above application(s) is hereby expresslyincorporated by reference, in its entirety, into the presentapplication.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a photoacoustic image generationapparatus comprising an insert of which at least a portion is insertedinto a subject and which includes a photoacoustic wave generationportion that absorbs light and generates photoacoustic waves.

2. Description of the Related Art

An ultrasonography method has been known as a kind of image inspectionmethod that can non-invasively inspect the internal state of a livingbody. In ultrasonography, an ultrasound probe that can transmit andreceive ultrasonic waves is used. In a case in which the ultrasoundprobe transmits ultrasonic waves to a subject (living body), theultrasonic waves travel in the living body and are reflected from theinterface between tissues. The ultrasound probe receives the reflectedultrasonic waves and a distance is calculated on the basis of the timeuntil the reflected ultrasonic waves return to the ultrasound probe. Inthis way, it is possible to capture an image indicating the internalaspect of the living body.

In addition, photoacoustic imaging has been known which captures theimage of the inside of a living body using a photoacoustic effect. Ingeneral, in the photoacoustic imaging, the inside of the living body isirradiated with pulsed laser light. In the inside of the living body, aliving body tissue absorbs the energy of the pulsed laser light andultrasonic waves (photoacoustic waves) are generated by adiabaticexpansion caused by the energy. For example, an ultrasound probe detectsthe photoacoustic waves and a photoacoustic image is formed on the basisof a detection signal. In this way, it is possible to visualize theinside of the living body on the basis of the photoacoustic waves.

In addition, as a technique related to the photoacoustic imaging,JP2015-231583A discloses a puncture needle in which a photoacoustic wavegeneration portion that absorbs light and generates photoacoustic wavesis provided in the vicinity of a tip. In the puncture needle, an opticalfiber is provided up to the tip of the puncture needle and light guidedby the optical fiber is emitted to the photoacoustic wave generationportion. An ultrasound probe detects the photoacoustic waves generatedby the photoacoustic wave generation portion and a photoacoustic imageis generated on the basis of a detection signal of the photoacousticwaves. In the photoacoustic image, a part of the photoacoustic wavegeneration portion appears as a bright point, which makes it possible tocheck the position of the puncture needle using the photoacoustic image.

SUMMARY OF THE INVENTION

In the photoacoustic imaging using the puncture needle as described inJP2015-231583A, it is suggested that the photoacoustic image indicatingthe position of the puncture needle is synthesized on a B modeultrasound image that displays the internal state of the living body ofthe subject using a two-dimensional image such that the tip position ofthe puncture needle in the inside of the living body can be easilychecked.

Here, it is possible to obtain the B mode ultrasound image with a highresolution by capturing the image using an ultrasound signal with a highfrequency, but the ultrasound with the high frequency is difficult toreach a deep part of the living body. Therefore, in JP2016-067552A, itis suggested that the image capturing is performed using the ultrasoundsignal with the high frequency in a shallow part of the living bodywhere attenuation of the ultrasound is low and the image capturing isperformed using the ultrasound signal with a low frequency in the deeppart of the living body where the attenuation of the ultrasound is highto acquire an image having as much information as possible.

In this case, for the photoacoustic image for specifying the tipposition of the puncture needle, in a case where the image capturing isperformed using a photoacoustic wave signal with the same frequency asthe generation of B mode ultrasound image, the resolution ofphotoacoustic image is also changed according to the depth of the livingbody tissue. In the photoacoustic image indicating the tip position ofthe puncture needle, the tip position of the puncture needle is commonlydisplayed by the bright point. However, since a display size of thebright point depends on a detection frequency (wavelength) of thephotoacoustic wave signal at the time of the generation of thephotoacoustic image and thus an image size for one wavelength, which isthe minimum resolution, becomes larger as the detection frequency islower (wavelength is longer), the tip position thereof is displayed by alarge bright point. Therefore, the display size of the bright pointindicating the tip position of the puncture needle according to thedepth of the living body tissue is changed, which may cause a user tofeel discomfort.

In order to solve such a problem, it may be considered that the imagecapturing is performed using the photoacoustic wave signal with aconstant frequency for the photoacoustic image for specifying the tipposition of the puncture needle. However, in this case, the imagecapturing is often performed using the photoacoustic wave signal withthe low frequency such that the tip position of the puncture needle canbe detected at any depth inside the living body.

In this case, in a case where, for example, only a high resolutionregion of the shallow part of the living body in the B mode ultrasoundimage is displayed with the high resolution (pixel/mm) and thephotoacoustic image with the low resolution is synthesized on the B modeultrasound image, there may be a problem that the display size of thebright point becomes large, thus a region of the B mode ultrasound imagehidden by the bright point becomes wide, and thus it is difficult toview the living body tissue around the tip of the puncture needle. Inthe puncture, since it is important whether the needle tip reaches theliving body tissue of interest such as a blood vessel or a tumor, it isdesired that the living body tissue around the tip of the punctureneedle can be clearly checked as much as possible.

Conversely, in a case where a wide region up to the deep part of theliving body in the B mode ultrasound image is displayed with the lowresolution (pixel/mm), there may be a problem that the display size ofthe bright point becomes too small and thus it is difficult to check thetip position of the puncture needle.

In this way, the optimum display size of the bright point in thephotoacoustic image is different according to the resolution in the caseof displaying the B mode ultrasound image. Therefore, it is desired thatit is possible to acquire the photoacoustic image with an appropriateresolution according to the resolution of the B mode ultrasound image.

The invention has been made in view of the above-mentioned problems andan object of the invention is to provide a photoacoustic imagegeneration apparatus capable of acquiring a photoacoustic image with anappropriate resolution according to a resolution of a B mode ultrasoundimage in the photoacoustic image generation apparatus that acquires andsynthesizes both the B mode ultrasound image and a photoacoustic image.

A photoacoustic image generation apparatus according to the inventioncomprises: an insert of which at least a tip portion is inserted into asubject and which includes a light guide member that guides light to thetip portion and a photoacoustic wave generation portion that absorbs thelight guided by the light guide member and generates photoacousticwaves; an acoustic wave detection unit that detects the photoacousticwaves emitted from the photoacoustic wave generation portion and detectsreflected acoustic waves reflected by transmission of acoustic waves tothe subject; an acoustic image generation unit that generates a B modeacoustic image on the basis of the reflected acoustic waves detected bythe acoustic wave detection unit; a photoacoustic image generation unitthat generates a photoacoustic image on the basis of the photoacousticwaves detected by the acoustic wave detection unit; an image output unitthat outputs a display image obtained by synthesizing the B modeacoustic image and the photoacoustic image; and a control unit thatcontrols a detection frequency of the photoacoustic waves used in a casewhere the photoacoustic image is generated in the photoacoustic imagegeneration unit on the basis of a resolution of the B mode acousticimage forming the display image.

Here, the “resolution of the B mode acoustic image forming the displayimage” means the number of assigned pixels per a predetermined length inthe display image and is represented by, for example, a unit ofpixel/mm.

In the photoacoustic image generation apparatus according to theinvention, the control unit may control the detection frequency of thephotoacoustic waves used in the case where the photoacoustic image isgenerated in the photoacoustic image generation unit to be higher as theresolution of the B mode acoustic image forming the display imagebecomes higher.

In the photoacoustic image generation apparatus according to theinvention, the control unit may decide the detection frequency of thephotoacoustic waves used in the case where the photoacoustic image isgenerated in the photoacoustic image generation unit such that a productof the resolution of the B mode acoustic image forming the display imageand a wavelength at the detection frequency of the photoacoustic wavesused in the case where the photoacoustic image is generated becomes apredetermined constant value.

In this case, it is preferable that the control unit may decide thepredetermined constant value on the basis of the minimum resolution ofthe B mode acoustic image and a detection lower limit frequency in theacoustic wave detection unit.

In the photoacoustic image generation apparatus according to theinvention, in a case where the detection frequency of the photoacousticwaves used in the case where the photoacoustic image is generated in thephotoacoustic image generation unit exceeds a predetermined upper limitvalue, the control unit may perform a control of deciding the detectionfrequency of the photoacoustic waves used in the case where thephotoacoustic image is generated in the photoacoustic image generationunit as the predetermined upper limit value and causing the image outputunit to reduce the photoacoustic image generated by setting thedetection frequency of the photoacoustic waves as the predeterminedupper limit value and synthesize the reduced image with the B modeacoustic image.

In this case, it is preferable that the predetermined upper limit valueis set to be equal to or less than a center frequency of the acousticwave detection unit.

In the photoacoustic image generation apparatus according to theinvention, a reference table holding unit that holds a reference tablehaving recorded thereon a relationship between the resolution of the Bmode acoustic image forming the display image and the detectionfrequency of the photoacoustic waves used in the case where thephotoacoustic image is generated in the photoacoustic image generationunit may be further provided. The control unit may decide the detectionfrequency of the photoacoustic waves used in the case where thephotoacoustic image is generated in the photoacoustic image generationunit on the basis of the reference table.

In the photoacoustic image generation apparatus according to theinvention, it is preferable that the acoustic wave detection unitalternately detects the reflected acoustic waves for generating the Bmode acoustic image and the photoacoustic waves for generating thephotoacoustic image, and the image output unit outputs one display imageon the basis of two images acquired in order of the B mode acousticimage and the photoacoustic image.

It is preferable that the insert is a needle that is inserted into thesubject.

The photoacoustic image generation apparatus according to the inventioncan acquire the photoacoustic image with the appropriate resolutionaccording to the resolution of the B mode ultrasound image since thedetection frequency of the photoacoustic waves used in the case wherethe photoacoustic image is generated is controlled on the basis of theresolution of the B mode acoustic image forming the display image in thephotoacoustic image generation apparatus that acquires both the B modeultrasound image and the photoacoustic image, synthesizes both images,and outputs the synthesized image as the display image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically illustrating the configurationof a first embodiment of a photoacoustic image generation apparatusaccording to the invention.

FIG. 2 is a cross-sectional view of the configuration of a tip portionof a puncture needle.

FIG. 3 is a diagram illustrating an example of a display image.

FIG. 4 is a diagram illustrating another example of the display image.

FIG. 5 is a graph illustrating a relationship between a detectionfrequency of photoacoustic waves and display depth.

FIG. 6 is a flowchart for describing a generation method of the displayimage in the photoacoustic image generation apparatus according to thefirst embodiment.

FIG. 7 is a block diagram schematically illustrating the configurationof a second embodiment of the photoacoustic image generation apparatusaccording to the invention.

FIG. 8 is a diagram illustrating an example of a reference table in thephotoacoustic image generation apparatus according to the secondembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a first embodiment of a photoacoustic image generationapparatus according to the invention will be described in detail withreference to the drawings. FIG. 1 is a block diagram schematicallyillustrating the configuration of the first embodiment of thephotoacoustic image generation apparatus according to the invention.

As illustrated in FIG. 1, a photoacoustic image generation apparatus 10according to this embodiment comprises an ultrasound probe 11, anultrasound unit 12, a laser unit 13, and a puncture needle 15. Thepuncture needle 15 and the laser unit 13 are connected by an opticalcable 16 having an optical fiber. The puncture needle 15 can be attachedto and detached from the optical cable 16 and is disposable. Inaddition, in this embodiment, ultrasonic waves are used as acousticwaves. However, the invention is not limited to the ultrasonic waves.Acoustic waves with an audible frequency may be used as long as anappropriate frequency can be selected according to, for example, aninspection target or measurement conditions.

The laser unit 13 comprises a solid-state laser light source using, forexample, yttrium aluminum garnet (YAG) and alexandrite. Laser lightemitted from the solid-state laser light source of the laser unit 13 isguided by the optical cable 16 and is incident on the puncture needle15. The laser unit 13 according to this embodiment emits pulsed laserlight in a near-infrared wavelength range. The near-infrared wavelengthrange means a wavelength range approximately from 700 nm to 850 nm. Inthis embodiment, the solid-state laser light source is used. However,other laser light sources, such as a gas laser light source, may be usedor light sources other than the laser light source may be used.

The puncture needle 15 is an embodiment of an insert according to theinvention and is a needle that is inserted into a subject. FIG. 2 is across-sectional view including a center axis that extends in a lengthdirection of the puncture needle 15. The puncture needle 15 includes apuncture needle main body 15 a that has an opening at an acute tip andis formed in a hollow shape, an optical fiber 15 b (corresponding to alight guide member according to the invention) that guides laser lightemitted from the laser unit 13 to the vicinity of the opening of thepuncture needle 15, and a photoacoustic wave generation portion 15 cthat absorbs laser light emitted from the optical fiber 15 b andgenerates photoacoustic waves.

The optical fiber 15 b and the photoacoustic wave generation portion 15c are disposed in a hollow portion 15 d of the puncture needle main body15 a. For example, the optical fiber 15 b is connected to the opticalfiber in the optical cable 16 (see FIG. 1) through an optical connectorthat is provided at the base end of the puncture needle 15. For example,laser light of 0.2 mJ is emitted from a light emission end of theoptical fiber 15 b.

The photoacoustic wave generation portion 15 c is provided at the lightemission end of the optical fiber 15 b and is provided in the vicinityof the tip of the puncture needle 15 and in the inner wall of thepuncture needle main body 15 a. The photoacoustic wave generationportion 15 c absorbs the laser light emitted from the optical fiber 15 band generates photoacoustic waves. The photoacoustic wave generationportion 15 c is made of, for example, an epoxy resin, a polyurethaneresin, a fluorine resin, and silicone rubber with which a black pigmentis mixed. In FIG. 2, the photoacoustic wave generation portion 15 c isillustrated to be larger than the optical fiber 15 b. However, theinvention is not limited thereto. The photoacoustic wave generationportion 15 c may have a size that is equal to the diameter of theoptical fiber 15 b.

The photoacoustic wave generation portion 15 c is not limited to theabove, and a metal film or an oxide film having light absorptivity withrespect to the wavelength of laser light may be used as thephotoacoustic wave generation portion. An oxide film made of, forexample, iron oxide, chromium oxide, or manganese oxide having highlight absorptivity with respect to the wavelength of laser light can beused as the photoacoustic wave generation portion 15 c. Alternatively, ametal film made of, for example, titanium (Ti) or platinum (Pt) that haslower light absorptivity than an oxide and has higher biocompatibilitythan an oxide may be used as the photoacoustic wave generation portion15 c. In addition, the position where the photoacoustic wave generationportion 15 c is provided is not limited to the inner wall of thepuncture needle main body 15 a. For example, a metal film or an oxidefilm which is the photoacoustic wave generation portion 15 c may beformed on the light emission end of the optical fiber 15 b with athickness of about 100 nm by vapor deposition such that the oxide filmcovers the light emission end. In this case, at least a portion of thelaser light emitted from the light emission end of the optical fiber 15b is absorbed by the metal film or the oxide film covering the lightemission end and photoacoustic waves are generated from the metal filmor the oxide film.

Returning to FIG. 1, the ultrasound probe 11 detects the photoacousticwaves emitted from the photoacoustic wave generation portion 15 c afterthe puncture needle 15 is inserted into the subject. The ultrasoundprobe 11 comprises an acoustic wave detection unit 20 that detects thephotoacoustic waves.

The acoustic wave detection unit 20 comprises a piezoelectric elementarray in which a plurality of piezoelectric elements that detect thephotoacoustic waves are one-dimensionally arranged and a multiplexer.The piezoelectric element is an ultrasound transducer, and theultrasound transducer is a piezoelectric element made of a polymer filmsuch as piezoelectric ceramics or polyvinylidene fluoride (PVDF). Theacoustic wave detection unit 20 comprises an acoustic lens, an acousticmatching layer, a backing member, a control circuit of the piezoelectricelement array, and the like.

With the piezoelectric element array of the acoustic wave detection unit20, the ultrasound probe 11 transmits the acoustic waves (ultrasonicwaves) to the subject and receives the reflected acoustic waves(reflected ultrasonic waves) with respect to the transmitted ultrasonicwaves, in addition to the detection of the photoacoustic waves. Inaddition, the transmission and reception of the ultrasonic waves may beperformed at different positions. For example, ultrasonic waves may betransmitted from a position different from the ultrasound probe 11, andthe piezoelectric element array of the ultrasound probe 11 may receivethe reflected ultrasonic waves with respect to the transmittedultrasonic waves. For example, a linear ultrasound probe, a convexultrasound probe, or a sector ultrasound probe may be used as theultrasound probe 11.

The ultrasound unit 12 includes the receiving circuit 21, a receivingmemory 22, a data demultiplexing unit 23, a photoacoustic imagegeneration unit 24, an ultrasound image generation unit 25, an imageoutput unit 26, a transmission control circuit 27, and a control unit28. The ultrasound unit 12 typically includes, for example, a processor,a memory, and a bus. In the ultrasound unit 12, a program related to,for example, a photoacoustic image generation process, an ultrasoundimage generation process, and a process of generating the display imageobtained by synthesizing the ultrasound image and the photoacousticimage is incorporated into a memory. The program is operated by thecontrol unit 28 which is formed by a processor to implement thefunctions of the data demultiplexing unit 23, the photoacoustic imagegeneration unit 24, the ultrasound image generation unit 25, and theimage output unit 26. That is, each of these units is formed by thememory into which the program has been incorporated and the processor.

The hardware configuration of the ultrasound unit 12 is not particularlylimited and can be implemented by combining, for example, a plurality ofintegrated circuits (ICs), a processor, an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), and amemory as appropriate.

The receiving circuit 21 receives a detection signal output from theultrasound probe 11 and stores the received detection signal in thereceiving memory 22. The receiving circuit 21 typically includes alow-noise amplifier, a variable-gain amplifier, a low-pass filter, andan analog-to-digital convertor (AD convertor). The detection signal ofthe ultrasound probe 11 is amplified by the low-noise amplifier, issubjected to gain adjustment corresponding to a depth by thevariable-gain amplifier, is converted into a digital signal by the ADconvertor after a high-frequency component of the detection signal iscut by the low-pass filter, and then is stored in the receiving memory22. The receiving circuit 21 is formed by, for example, one integratedcircuit (IC).

The ultrasound probe 11 outputs a detection signal of the photoacousticwaves and a detection signal of the reflected ultrasonic waves. Thereceiving memory 22 stores the AD-converted detection signals (samplingdata) of the photoacoustic waves and the reflected ultrasonic waves. Thedata demultiplexing unit 23 reads the detection signal of thephotoacoustic waves from the receiving memory 22 and transmits thedetection signal to the photoacoustic image generation unit 24. The datademultiplexing unit 23 reads the detection signal of the reflectedultrasonic waves from the receiving memory 22 and transmits thedetection signal to the ultrasound image generation unit 25.

The photoacoustic image generation unit 24 generates a photoacousticimage on the basis of the detection signal of the photoacoustic wavesdetected by the ultrasound probe 11. The photoacoustic image generationprocess includes, for example, image reconfiguration such as phasingaddition, detection, and logarithmic conversion. The ultrasound imagegeneration unit 25 generates a B mode ultrasound image (corresponding toa B mode acoustic image according to the invention) that displays theinternal state of the living body of the subject using a two-dimensionalimage on the basis of the detection signal of the reflected ultrasonicwaves detected by the ultrasound probe 11. The B mode ultrasound imagegeneration process also includes image reconfiguration such as phasingaddition, detection, and logarithmic conversion. The image output unit26 outputs the photoacoustic image and the B mode ultrasound image on animage display unit 30 such as a display apparatus.

The control unit 28 controls each component in the ultrasound unit 12.For example, in a case in which a photoacoustic image is acquired, thecontrol unit 28 transmits a trigger signal to the laser unit 13 suchthat the laser unit 13 emits laser light. In addition, the control unit28 transmits a sampling trigger signal to the receiving circuit 21 tocontrol, for example, the sampling start time of the photoacoustic waveswith the emission of the laser light. Sampling data received by thereceiving circuit 21 is stored in the receiving memory 22.

The photoacoustic image generation unit 24 receives the sampling data ofthe detection signal of the photoacoustic waves through the datademultiplexing unit 23 and performs detection at a predetermineddetection frequency to generate the photoacoustic image. Thephotoacoustic image generated by the photoacoustic image generation unit24 is input to the image output unit 26.

In a case in which a B mode ultrasound image is acquired, the controlunit 28 transmits an ultrasound transmission trigger signal forcommanding the transmission control circuit 27 to transmit theultrasonic waves. In a case in which the ultrasound transmission triggersignal is received, the transmission control circuit 27 causes theultrasound probe 11 to transmit ultrasonic waves. In a case in which theultrasound image is acquired, the ultrasound probe 11 performs ascanning while shifting a reception region of a group of piezoelectricelements line by line to detect the reflected ultrasonic waves by thecontrol of the control unit 28. The control unit 28 transmits thesampling trigger signal to the receiving circuit 21 according to thetransmission time of ultrasonic waves to start the sampling of thereflected ultrasonic waves. The sampling data received by the receivingcircuit 21 is stored in the receiving memory 22.

The ultrasound image generation unit 25 receives the sampling data ofthe detection signal of the photoacoustic waves through the datademultiplexing unit 23 and performs the detection at a predetermineddetection frequency to generate the B mode ultrasound image. The B modeultrasound image generated by the ultrasound image generation unit 25 isinput to the image output unit 26.

The image output unit 26 synthesizes the photoacoustic image generatedby the photoacoustic image generation unit 24 and the B mode ultrasoundimage generated by the ultrasound image generation unit 25 to generatethe display image and outputs the generated display image on the imagedisplay unit 30 such as a display apparatus. The image output unit 26can individually output and display the photoacoustic image and the Bmode ultrasound image on the image display unit 30 without synthesizingboth images.

Here, a generation method of the display image by the control unit 28will be described in detail. FIG. 3 is a diagram illustrating an exampleof the display image, and FIG. 4 is a diagram illustrating anotherexample of the display image.

As illustrated in FIG. 3, a display image I_(s) is generated bysuperimposing a photoacoustic image I_(n) on a B mode ultrasound imageI_(b). In this embodiment, it is assumed that one display image I_(s) isgenerated on the basis of two images acquired in order of the B modeultrasound image I_(b) and the photoacoustic image I_(n).

It is assumed that the detection frequency at the time of the generationof the B mode ultrasound image I_(b) is controlled according to thedepth of the living body tissue of the subject. The detection frequencyat the time of the generation of the photoacoustic image I_(n) has afixed initial value according to the depth, but the value is changed bythe control at the time of the generation of the display image I_(s)described below. In this description, it is assumed that f_(b) is thedetection frequency (MHz) at the time of the generation of the B modeultrasound image I_(b), R_(b) is the resolution (pixel/mm) of the B modeultrasound image I_(b), and f_(n) is the detection frequency (MHz) atthe time of the generation of the photoacoustic image I_(n).

For example, in a case where the sound speed c in the living body is1500 m/s (normally, the sound speed in the living body is about 1540m/s, but it is simplified herein), the initial value f_(n) of thedetection frequency at the time of the generation of the photoacousticimage I_(n) is 5 MHz (wavelength λ_(n)=300 μm), and the number of pixelsin the longitudinal direction of an image display region of the imagedisplay unit 30 is 400 pixels, and as illustrated in FIG. 3, a displayimage I_(s) of a living body tissue having 3 cm (30 mm) in the lateraldirection×2 cm (20 mm) in the depth direction is displayed such that adepth display direction of the display image I_(s) is the entirelongitudinal direction of the image display region of the image displayunit 30, the resolution R_(b) of the B mode ultrasound image I_(b) is 20pixels/mm as follows.

R _(b)=400 (pixels)/20 (mm)=20 (pixels/mm)

The number of pixels on which one wavelength of the detection frequencyat the time of the generation of the photoacoustic image I_(n) isdisplayed is six pixels as follows.

20 (pixels/mm)×0.3 (mm)=6 (pixels)

As illustrated in FIG. 4, in a case where a display image I_(s) of aliving body tissue having 3 cm (30 mm) in the lateral direction×4 cm (40mm) in the depth direction is displayed such that the depth displaydirection of the display image I_(s) is the entire longitudinaldirection of the image display region of the image display unit 30, theresolution R_(b) of the B mode ultrasound image I_(b) is 10 pixels/mm asfollows.

R _(b)=400 (pixels)/40 (mm)=10 (pixels/mm)

The number of pixels on which one wavelength of the detection frequencyat the time of the generation of the photoacoustic image I_(n) isdisplayed is three pixels as follows.

10 (pixels/mm)×0.3 (mm)=3 (pixels)

A setting of a display range of the display image I_(s) as describedabove is input to the control unit 28 through an input unit 40 by auser.

In this way, in a case where there is a difference in the resolution ofthe display image I_(s), a tradeoff occurs between f_(n) and R_(b) in acase where the detection frequency f_(n) at the time of the generationof the photoacoustic image I_(n) is a fixed value. As illustrated inFIGS. 3 and 4, the tip position of the puncture needle 15 is commonlydisplayed by a bright point P in the photoacoustic image I_(n)indicating the tip position of the puncture needle 15. However, sincethe display size of the bright point P depends on the detectionfrequency (wavelength) of the photoacoustic wave signal at the time ofthe generation of the photoacoustic image I_(n) and thus the image sizefor one wavelength, which is the minimum resolution, becomes larger asthe detection frequency is lower (wavelength is longer), the tipposition thereof is displayed by a large bright point P.

In a case where the diameter of about three pixels illustrated in FIG. 4is considered to be optimum as the display size of the bright point P,the detection frequency f_(n) at the time of the generation of thephotoacoustic image I_(n) becomes too small with respect to theresolution R_(b) of the B mode ultrasound image I_(b) in the exampleillustrated in FIG. 3 and thus the display size of the bright point Pbecomes too large at the diameter of about six pixels. Therefore, theremay be a problem that a region of the B mode ultrasound image I_(b)hidden by the bright point P becomes wide and thus it is difficult toview the living body tissue around the tip of the puncture needle 15.

Conversely, in a case where the detection frequency f_(n) at the time ofthe generation of the photoacoustic image I_(n) becomes too large withrespect to the resolution R_(b) of the B mode ultrasound image I_(b),there may be a problem that the display size of the bright point Pbecomes too small and thus it is difficult to check the tip position ofthe puncture needle 15. For example, in a case where the resolutionR_(b) of the B mode ultrasound image I_(b) is 10 pixels/mm similar tothe example of FIG. 4 and the detection frequency f_(n) at the time ofthe generation of the photoacoustic image I_(n) is changed to 10 MHz(wavelength λ_(n)=150 μm), the number of pixels on which one wavelengthof the detection frequency at the time of the generation of thephotoacoustic image I_(n) is displayed is 1.5 pixels.

That is, there is an optimum detection frequency f_(n) at the time ofthe generation of the photoacoustic image I_(n) for image display withrespect to the resolution R_(b) of the B mode ultrasound image I_(b).

In the photoacoustic image generation apparatus 10 according to thisembodiment, the detection frequency f_(n) at the time of the generationof the photoacoustic image I_(n) is decided such that the product of theresolution R_(b) of the B mode ultrasound image I_(b) and the wavelengthλ_(n) at the detection frequency f_(n) at the time of the generation ofthe photoacoustic image I_(n) is a predetermined constant value (const)as described below. That is, the detection frequency f_(n) at the timeof the generation of the photoacoustic image I_(n) is controlled to behigher as the resolution R_(b) of the B mode ultrasound image I_(b)becomes higher. Here, α is a bright point display size adjustmentparameter.

R _(b)×λ_(n)×α=(const)

In a case where the above equation is rewritten as an equation of thedetection frequency f_(n) at the time of the generation of thephotoacoustic image I_(n), the following equation (1) is obtained.

R _(b) ×c/f _(n)=(const)/α

f _(n) =R _(b) ×c×α/(const)+δ  (1)

Since the detection frequency f_(n) that can be handled by the apparatushas a discrete value due to the limitation of the number of clocks,there may be a case where f_(n) cannot be a value (here, f) obtainedfrom equation (1) excluding δ. The δ in equation (1) is an adjustmentparameter for adjusting such an error and is obtained as follows. Here,Mod[frx_clock,f] is the remainder at the time of dividing a receptionclock frequency frx_clock by f.

δ=f _(n) ×f×Mod[frx_clock,f]

In this way, the detection frequency f_(n) at the time of the generationof the photoacoustic image I_(n) is decided according to the resolutionR_(b) of the B mode ultrasound image I_(b) by equation (1). Thus, it ispossible to make the display size of the bright point P constant withoutdepending on the change in the resolution R_(b).

As described above, in the case where the detection frequency f_(b) atthe time of the generation of the B mode ultrasound image I_(b) iscontrolled according to the depth of the living body tissue of thesubject, the detection frequency f_(b) is decided by a transmissionfrequency in the ultrasound probe 11 and a depth position of the livingbody tissue. Basically, for the detection frequency f_(b), thetransmission frequency is used in the shallowest region in a detectionrange and a lower limit frequency of the ultrasound probe 11 is used inthe deepest region in the detection range. Therefore, an upper limitvalue of the detection frequency f_(b) is decided by a transmissioncondition, and a lower limit value thereof is decided by the performanceof the ultrasound probe 11.

Since there is no transmission from the ultrasound probe 11 and there isonly reception in the ultrasound probe 11 at the time of the generationof the photoacoustic image I_(n), it is desirable that the lower limitvalue of the detection frequency f_(b) is uniquely decided by theperformance of the ultrasound probe 11.

Therefore, it is preferable that the predetermined constant value(const) is decided on the basis of the minimum resolution R_(b_MIN) ofthe B mode ultrasound image I_(b) and a detection lower limit frequencyf_(MIN) in the ultrasound probe 11.

(const)=R _(b_MIN) ×c/f _(MIN)×α

Here, the detection lower limit frequency f_(MIN) indicates the minimumvalue of the detection frequency f_(b) at the time of the generation ofthe B mode ultrasound image I_(b). Since the same reception signal fromthe ultrasound probe 11 is used also for the photoacoustic image I_(n),the minimum value of the detection frequency is the lowest frequencypart of a frequency band (for example, −20 dB band) usable by theultrasound probe 11 and the value is the same value as the B modeultrasound image I_(b).

This point will be described with a specific example. FIG. 5 is a graphillustrating a relationship between the detection frequency of thephotoacoustic waves and the display depth. In a case where the maximumdetection depth of the living body tissue of the subject is 8 cm and adisplay image I_(s) of a living body tissue having 8 cm (80 mm) in thedepth direction is displayed such that the depth display direction ofthe display image I_(s) is the entire longitudinal direction (400pixels) of the image display region of the image display unit 30 similarto the above, the minimum resolution R_(b_MIN) of the B mode ultrasoundimage I_(b) is 5 pixels/mm as follows. In a case where the detectionlower limit frequency f_(MIN) is 5 MHz (wavelength λ_(n)=300 μm) and thebright point display size adjustment parameter α is one, the displaysize (const) of the bright point P is 1.5 pixels.

(const)=5 (pixels/mm)×0.3 (mm)×1=1.5 (pixels)

There may also be a case where the display depth exceeds 16 cm or 20 cmdepending on a type of the ultrasound probe 11 and an observation target(such as abdomen). In a case where the number of pixels (400 pixels) inthe entire longitudinal direction of the image display region of theimage display unit 30 and the detection lower limit frequency f_(MIN) of5 MHz (wavelength λ_(n)=300 μm) are the same, the display size (const)of the bright point P is 0.75 pixel in the case of the maximum displaydepth of 16 cm and the display size (const) of the bright point P is 0.6pixel in the case of the maximum display depth of 20 cm. In the casewhere the display size of the bright point P becomes too small asdescribed above, it is possible to adjust the display size of the brightpoint P by adjusting the bright point display size adjustment parameterα.

Since the bright point P indicating the tip position of the punctureneedle 15 in the photoacoustic image I_(n) is displayed as a point, itis important to grasp the position surely rather than acquiring detailedimage information. Since a signal with high depth reachability can bereceived at a lower frequency as the detection frequency at the time ofthe generation of the photoacoustic image I_(n), a stable image displayis possible.

Therefore, in a case where the detection frequency f_(n) at the time ofthe generation of the photoacoustic image I_(n) obtained by equation (1)exceeds a predetermined upper limit value f_(n_MAX) as illustrated inthe graph of FIG. 5, it is preferable that the detection frequency ofthe photoacoustic waves used in the case where the photoacoustic imageI_(n) is generated is decided as f_(n_MAX) and the photoacoustic imageI_(n) generated by setting the detection frequency of the photoacousticwaves as f_(n_MAX) is reduced and synthesized with the B mode ultrasoundimage.

In this case, it is preferable that the predetermined upper limit valuef_(n_MAX) is set to be equal to or less than a center frequency of theultrasound probe 11. Accordingly, a stable image display is possible.

A reduction ratio (image size after reduction/image size beforereduction) R_(R) in the case where the photoacoustic image I_(n)generated by setting the detection frequency of the photoacoustic wavesas f_(n_MAX) is reduced is obtained by the following equation (2).

R _(R) =f _(n_MAX) /f _(n)  (2)

In the case of reducing the photoacoustic image I_(n), the reduction isperformed by setting the center position of the bright point P in thephotoacoustic image I_(n) as the center. Since the B mode ultrasoundimage I_(b) becomes different from the photoacoustic image I_(n) in theimage size in the case where the photoacoustic image I_(n) is reduced,it is impossible to simply synthesize the B mode ultrasound image I_(b)and the photoacoustic image I_(n).

In a case where the reduced photoacoustic image I_(n) is synthesizedwith the B mode ultrasound image I_(b), a center position coordinate ofthe bright point P in the case of superimposing the photoacoustic imageI_(n) on the B mode ultrasound image I_(b) and an image around thebright point P may be needed. Therefore, only these pieces ofinformation may be acquired.

For the image around the bright point P, for example, in a case wherethe display size (const) of the bright point P is three pixels and thedetection frequency f_(n) of the photoacoustic waves used in the casewhere the photoacoustic image I_(n) is generated is suppressed tof_(n_MAX) which is the frequency lower than the detection frequencyf_(n) obtained by equation (1), the display size of the bright point Pin the photoacoustic image I_(n) acquired at the detection frequencyf_(n_MAX) becomes larger than three pixels. Therefore, an image rangearound the bright point may be set slightly wider in consideration ofthe reduction ratio R_(R). For example, in a case where the reductionratio R_(R) is 0.5 with respect to the final display size (const) of thebright point P of three pixels, only an image having a region of sixpixels×six pixels may be acquired.

On the basis of the above, in this embodiment, the generation of thedisplay image I_(s) by the control unit 28 is performed according to aprocedure of a flowchart illustrated in FIG. 6.

First, the control unit 28 calculates the detection frequency f_(n) atthe time of the generation of the photoacoustic image I_(n) according tothe resolution R_(b) of the B mode ultrasound image I_(b) by equation(1) described above (S1) and determines whether the detection frequencyf_(n) obtained by equation (1) exceeds the upper limit value (S2).

In a case where the calculated detection frequency f_(n) does not exceedthe predetermined upper limit value, the control unit 28 causes thephotoacoustic image generation unit 24 to generate the photoacousticimage I_(n) at the calculated detection frequency f_(n) (S3) and causesthe image output unit 26 to output the display image I_(s) obtained bysynthesizing the B mode ultrasound image I_(b) and the photoacousticimage I_(n) (S4).

In a case where the calculated detection frequency f_(n) exceeds thepredetermined upper limit value, the control unit 28 calculates thereduction ratio of the photoacoustic image I_(n) by equation (2)described above (S5), causes the photoacoustic image generation unit 24to generate the photoacoustic image I_(n) at the detection frequencyf_(n_MAX) of the predetermined upper limit value, causes the imageoutput unit 26 to perform a reduction process on the photoacoustic imageI_(n) with the reduction ratio obtained by equation (2) (S6), and causesthe image output unit 26 to output the display image I_(s) obtained bysynthesizing the B mode ultrasound image I_(b) and the reducedphotoacoustic image I_(n) (S4).

With such a configuration, an appropriate detection frequency f_(n) atthe time of the generation of the photoacoustic image I_(n) can be setaccording to the resolution R_(b) of the B mode ultrasound image I_(b)and the photoacoustic image I_(n) with an appropriate resolution can beacquired. Therefore, it is possible to set the size of the bright pointP in the photoacoustic image I_(n) superimposed on the B mode ultrasoundimage I_(b) to an appropriate size.

Next, a second embodiment of the photoacoustic image generationapparatus according to the invention will be described. In thephotoacoustic image generation apparatus 10 according to the firstembodiment, the detection frequency f_(n) at the time of the generationof the photoacoustic image I_(n) according to the resolution R_(b) ofthe B mode ultrasound image I_(b) is decided by the predeterminedcalculation equation. However, in the photoacoustic image generationapparatus 10 according to the second embodiment, the detection frequencyf_(n) at the time of the generation of the photoacoustic image I_(n)according to the resolution R_(b) of the B mode ultrasound image I_(b)is decided on the basis of a reference table prepared in advance. Otherconfigurations and actions are the same as those in the photoacousticimage generation apparatus 10 according to the first embodiment. FIG. 7is a block diagram schematically illustrating the configuration of thesecond embodiment of the photoacoustic image generation apparatusaccording to the invention. FIG. 8 is a diagram illustrating an exampleof the reference table in the photoacoustic image generation apparatusaccording to the second embodiment.

The photoacoustic image generation apparatus 10 according to the secondembodiment comprises a reference table holding unit 29 that holds areference table, such as the reference table illustrated in FIG. 8 as anexample, recording a relationship between the resolution R_(b) of the Bmode ultrasound image I_(b) and the detection frequency f_(n) at thetime of the generation of the photoacoustic image I_(n). The referencetable holding unit 29 can be implemented by, for example, a memory orthe like. In the case of generating the display image I_(s), the controlunit 28 decides the detection frequency f_(n) at the time of thegeneration of the photoacoustic image I_(n) on the basis of thereference table held in the reference table holding unit 29. With such aconfiguration, it is also possible to obtain the same effect as thephotoacoustic image generation apparatus 10 according to the firstembodiment.

In the above-described first and second embodiments, the puncture needle15 is used as an embodiment of the insert. However, the invention is notlimited thereto as the insert. The insert may be a radio-frequencyablation needle including an electrode that is used for radio-frequencyablation therein, a catheter that is inserted into a blood vessel, or aguide wire for a catheter that is inserted into a blood vessel.Alternatively, the insert may be an optical fiber for laser treatment.

The insert is not limited to a needle, such as an injection needle, andmay be a biopsy needle used for biopsy. That is, the needle may be abiopsy needle that is inserted into an inspection target of the livingbody and extracts the tissues of a biopsy site of the inspection target.In this case, photoacoustic waves may be generated from an extractionportion (intake port) for sucking and extracting the tissues of thebiopsy site. In addition, the needle may be used as a guiding needlethat is used for insertion into a deep part, such as a part under theskin or an organ inside the abdomen.

The invention has been described above on the basis of the preferredembodiments. However, the photoacoustic image generation apparatusesaccording to the invention are not limited only to the above-describedembodiments. Various modifications and changes of the configurationsaccording to the above-described embodiments are also included in thescope of the invention.

EXPLANATION OF REFERENCES

-   -   10: photoacoustic image generation apparatus    -   11: ultrasound probe    -   12: ultrasound unit    -   13: laser unit    -   15: puncture needle    -   15 a: puncture needle main body    -   15 b: optical fiber    -   15 c: photoacoustic wave generation portion    -   15 d: hollow portion    -   16: optical cable    -   20: acoustic wave detection unit    -   21: receiving circuit    -   22: receiving memory    -   23: data demultiplexing unit    -   24: photoacoustic image generation unit    -   25: ultrasound image generation unit    -   26: image output unit    -   27: transmission control circuit    -   28: control unit    -   29: reference table holding unit    -   30: image display unit    -   40: input unit    -   I_(b): B mode ultrasound image    -   I_(n): photoacoustic image    -   I_(s): display image    -   P: bright point

What is claimed is:
 1. A photoacoustic image generation apparatuscomprising: an insert of which at least a tip portion is inserted into asubject and which includes a light guide that guides light to the tipportion and a photoacoustic wave generation portion that absorbs thelight guided by the light guide and generates photoacoustic waves; anultrasound probe that detects the photoacoustic waves emitted from thephotoacoustic wave generation portion and detects reflected acousticwaves reflected by transmission of acoustic waves to the subject; and aprocessor, wherein the processor configured to: generate a B modeacoustic image on the basis of the reflected acoustic waves detected bythe ultrasound probe, generate a photoacoustic image on the basis of thephotoacoustic waves detected by the ultrasound probe, output a displayimage obtained by synthesizing the B mode acoustic image and thephotoacoustic image, and control a detection frequency of thephotoacoustic waves used in a case where the photoacoustic image isgenerated on the basis of a resolution of the B mode acoustic imageforming the display image.
 2. The photoacoustic image generationapparatus according to claim 1, wherein the processor further configuredto control the detection frequency of the photoacoustic waves used inthe case where the photoacoustic image is generated to be higher as theresolution of the B mode acoustic image forming the display imagebecomes higher.
 3. The photoacoustic image generation apparatusaccording to claim 1, wherein the processor further configured to decidethe detection frequency of the photoacoustic waves used in the casewhere the photoacoustic image is generated such that a product of theresolution of the B mode acoustic image forming the display image and awavelength at the detection frequency of the photoacoustic waves used inthe case where the photoacoustic image is generated becomes apredetermined constant value.
 4. The photoacoustic image generationapparatus according to claim 2, wherein the processor further configuredto decide the detection frequency of the photoacoustic waves used in thecase where the photoacoustic image is generated such that a product ofthe resolution of the B mode acoustic image forming the display imageand a wavelength at the detection frequency of the photoacoustic wavesused in the case where the photoacoustic image is generated becomes apredetermined constant value.
 5. The photoacoustic image generationapparatus according to claim 3, wherein the processor further configuredto decide the predetermined constant value on the basis of the minimumresolution of the B mode acoustic image and a detection lower limitfrequency in the ultrasound probe.
 6. The photoacoustic image generationapparatus according to claim 4, wherein the processor further configuredto decide the predetermined constant value on the basis of the minimumresolution of the B mode acoustic image and a detection lower limitfrequency in the ultrasound probe.
 7. The photoacoustic image generationapparatus according to claim 3, wherein in a case where the detectionfrequency of the photoacoustic waves used in the case where thephotoacoustic image is generated exceeds a predetermined upper limitvalue, the processor further configured to decide the detectionfrequency of the photoacoustic waves used in the case where thephotoacoustic image is generated as the predetermined upper limit value,reduce the photoacoustic image generated by setting the detectionfrequency of the photoacoustic waves as the predetermined upper limitvalue, and synthesize the reduced image with the B mode acoustic image.8. The photoacoustic image generation apparatus according to claim 4,wherein in a case where the detection frequency of the photoacousticwaves used in the case where the photoacoustic image is generatedexceeds a predetermined upper limit value, the processor furtherconfigured to decide the detection frequency of the photoacoustic wavesused in the case where the photoacoustic image is generated as thepredetermined upper limit value, reduce the photoacoustic imagegenerated by setting the detection frequency of the photoacoustic wavesas the predetermined upper limit value, and synthesize the reduced imagewith the B mode acoustic image.
 9. The photoacoustic image generationapparatus according to claim 5, wherein in a case where the detectionfrequency of the photoacoustic waves used in the case where thephotoacoustic image is generated exceeds a predetermined upper limitvalue, the processor further configured to decide the detectionfrequency of the photoacoustic waves used in the case where thephotoacoustic image is generated as the predetermined upper limit value,reduce the photoacoustic image generated by setting the detectionfrequency of the photoacoustic waves as the predetermined upper limitvalue, and synthesize the reduced image with the B mode acoustic image.10. The photoacoustic image generation apparatus according to claim 6,wherein in a case where the detection frequency of the photoacousticwaves used in the case where the photoacoustic image is generatedexceeds a predetermined upper limit value, the processor furtherconfigured to decide the detection frequency of the photoacoustic wavesused in the case where the photoacoustic image is generated as thepredetermined upper limit value, reduce the photoacoustic imagegenerated by setting the detection frequency of the photoacoustic wavesas the predetermined upper limit value, and synthesize the reduced imagewith the B mode acoustic image.
 11. The photoacoustic image generationapparatus according to claim 7, wherein the predetermined upper limitvalue is set to be equal to or less than a center frequency of theultrasound probe.
 12. The photoacoustic image generation apparatusaccording to claim 8, wherein the predetermined upper limit value is setto be equal to or less than a center frequency of the ultrasound probe.13. The photoacoustic image generation apparatus according to claim 9,wherein the predetermined upper limit value is set to be equal to orless than a center frequency of the ultrasound probe.
 14. Thephotoacoustic image generation apparatus according to claim 10, whereinthe predetermined upper limit value is set to be equal to or less than acenter frequency of the ultrasound probe.
 15. The photoacoustic imagegeneration apparatus according to claim 1, further comprising: areference table memory that holds a reference table having recordedthereon a relationship between the resolution of the B mode acousticimage forming the display image and the detection frequency of thephotoacoustic waves used in the case where the photoacoustic image isgenerated, wherein the processor further configured to decide thedetection frequency of the photoacoustic waves used in the case wherethe photoacoustic image is generated on the basis of the referencetable.
 16. The photoacoustic image generation apparatus according toclaim 2, further comprising: a reference table memory that holds areference table having recorded thereon a relationship between theresolution of the B mode acoustic image forming the display image andthe detection frequency of the photoacoustic waves used in the casewhere the photoacoustic image is generated, wherein the processorfurther configured to decide the detection frequency of thephotoacoustic waves used in the case where the photoacoustic image isgenerated on the basis of the reference table.
 17. The photoacousticimage generation apparatus according to claim 1, wherein the ultrasoundprobe alternately detects the reflected acoustic waves for generatingthe B mode acoustic image and the photoacoustic waves for generating thephotoacoustic image, and wherein the processor further configured tooutput one display image on the basis of two images acquired in order ofthe B mode acoustic image and the photoacoustic image.
 18. Thephotoacoustic image generation apparatus according to claim 2, whereinthe ultrasound probe alternately detects the reflected acoustic wavesfor generating the B mode acoustic image and the photoacoustic waves forgenerating the photoacoustic image, and wherein the processor furtherconfigured to output one display image on the basis of two imagesacquired in order of the B mode acoustic image and the photoacousticimage.
 19. The photoacoustic image generation apparatus according toclaim 3, wherein the ultrasound probe alternately detects the reflectedacoustic waves for generating the B mode acoustic image and thephotoacoustic waves for generating the photoacoustic image, and whereinthe processor further configured to output one display image on thebasis of two images acquired in order of the B mode acoustic image andthe photoacoustic image.
 20. The photoacoustic image generationapparatus according to claim 1, wherein the insert is a needle that isinserted into the subject.